This invention relates to an electromagnetic sensor system, and is particularly concerned with dealing with various types of noise which can be received by such a system. The system may be adapted for short range obstacle sensing. Such a system may be installed on a motor road vehicle as part of a collision warning system.
A known type of electromagnetic sensor system uses a series of broad band radio frequency pulses to detect the presence and/or motion of objects. Such a system has a pulse generator forming part of a transmitter for transmitting a train of radio frequency pulses. Echoes of those pulses are received by a receiving antenna, the output of which is sampled by a sampler at a succession of sampling periods, each occurring at a predetermined delay after the transmission of a respective pulse. If the reflection of a pulse is received during a given sampling period, this is indicative of the pulse having travelled to the object and returned to the receiver in the predetermined delay, so that it can be deduced that the object is entering or leaving a notional range gate or envelope surrounding the transmitter and receiver.
Examples of such systems are shown in U.S. Pat. No. 5,361,070 (McEwan) and European Patent No EP 469027B (Cambridge Consultants Limited).
In general, the magnitude of the reflected pulses can be small in relation to background noise, and as a result the signals received over the sampling periods can be averaged in order to improve the signal to noise ratio, as is discussed in U.S. Pat. No. 5,361,070.
However such a system is still susceptible to interference from RF spike noise (produced by other systems of the same type, for example) and continuous wave RF signals.
These latter problems are particularly relevant where the system is to be installed in a motor road vehicle, other vehicles may be equipped with similar systems, which generate the noise spikes, and the system is likely to be operated in the vicinity of various sources of continuous wave signals, such as mobile telephone apparatus/vehicle identification systems.
The signals strengths of an echo received by an electromagnetic sensor system from a target of cross-section e at range R is given by the following expression:
S=PGA"sgr"/(4xcfx80R2)2
where PGA is power-gain-area product for the system.
If the target is equipped with a similar system, then there is the possibility that a pulse transmitted by the target will arrive at the receiver during a sampling period. The signal strength of such an interfering pulse is:
I=PGA/(4xcfx80R2)
so that the signal-to-noise ratio, SNR, is:
SNR="sgr"/(4xcfx80R2)
For an automotive application typical values might be "sgr"=0.1 m2xc2x7R=30 m giving:
SNR=xe2x88x9250 dB
compared with a required signal-to-noise ratio typically of at least +15 dB for acceptable detection performance.
This signal-to-noise ratio will be improved by averaging the signals received over a large number of sampling periods, but even if 104 samples are averaged, the processing gain will only be 40 dB. Even if only one interfering pulse is received per averaging period, the averaged SNR will be xe2x88x9210 dB which might still be too low.
An impulse modulated electromagnetic sensor system operates at a pulse repetition frequency (PRF) typically of order 1-10 MHz: both the pulse generator in the transmitter and the sampler in the receiver will operate at this frequency. The audio frequency (AF) output from the sampler will have a bandwidth from DC to the Nyquist frequency (one half of the PRF).
For many applications, and in particular for automotive applications, the target echo occupies only a fraction of the AF bandwidth. The echo bandwidth is related to the wavelength of the signature (typically 3 to 30 cm) and the relative speed of the target (say 0 to 100 m/s) from which the pulses are reflected, giving a bandwidth of order 3.3 kHz. The output signal processing can include a low-pass filtering stage to improve the signal-to-noise ratio by rejecting noise (eg thermal noise in the receiver) outside the AF band.
The receive antenna will pick up any external RF signal, for example from a nearby radio transmitter. Of particular concern in automotive applications are on-car or roadside transmitters such as mobile phones or tolling vehicle identification systems. RF filters in the receiver can suppress any signal outside the operational bandwidth of the system, but other signals will be aliased into the AF output. For example, if PRF is 1 MHz and the operational bandwidth includes frequencies around 2GHz, then a signal at 2.000001 GHz will be aliased to 1 kHz and a signal at 2.000100 GHz will be aliased to 100 kHz. The low-pass filter (presumed DC to 3.3 kHz) in the output processing will reject the second signal but not the first.
In practice, modulated RF signals have finite bandwidth, typically 100 kHz for GSM systems, with corresponding channel separations of 200 kHz. An FM signal with a centre frequency of 2 GHz will appear in the AF output as energy distributed over the DCxe2x88x9250 kHz band, and a proportion of this energy will be passed by the low-pass filter. A signal with a centre frequency around 2.0002 GHz will appear in the AF output as energy distributed over 150-250 kHz, and should be rejected by the low pass filter.
Thus, in an impulse modulated electromagnetic sensor system the reflected signature from a target can be contaminated by continuous wave (CW) or amplitude/frequency /phase-modulated continuous wave (narrowband) RF signals which appear in the AF output. If their amplitude is sufficiently large they will obscure the wanted echo and prevent the target from being detected.
The invention seeks to provide various methods and apparatuses which are less susceptible to interference by either or both these types of noise.
According to a first aspect of the invention, there is provided an electromagnetic sensor system comprising transmitting means for transmitting a train of radio frequency pulses, receiving means for receiving reflections of said pulses from remote objects, sampling means for sampling the output of the receiving means, processing means connected to the sampling means, and operable to detect said reflections in the sampled signal, and to determine information on the presence or range of said object, and gating means for preventing radio pulses transmitted by other sources or noise spikes from causing interference which results in spurious detections or indications of range by the processing means.
Thus, a number of such systems may be used in proximity to each other since the gating means prevents the pulses transmitted by one of the systems being mistaken by another system for reflections of pulses transmitted by that other system. Such mistakes would otherwise give rise to spurious detections or other inaccuracies in the output of the processing means.
The gating means may be arranged to operate by monitoring signals received by the receiving means, and preventing or inhibiting the operation of the sampling means when the amplitude of said signals exceeds a threshold.
As is explained above, reflections of signals transmitted by the transmitting means will make a smaller contribution to the amplitude of the output of the receiving means than will transmitted pulses received directly, (i.e. without an intervening reflection) from other systems. Consequently, the above thresholding procedure will discriminate between most genuine reflections and other pulses, which are transmitted by other systems and which are received directly by the receiving means.
Preferably, the gating means comprises a threshold detector for determining whether the received signal is above said threshold and, if it is not, generating an enabling signal for enabling the sampler to operate, and delay means for delaying the passage of the received signal from the receiving means to the sampling means so that the operation of the threshold detector and the arrival of the signal at the sampling means are synchronised.
Alternatively, the filter means may be operable selectively to limit the amplitude of the input to the sampling means by truncating any peaks in the sampled signal which would otherwise exceed said threshold, so that the maximum amplitude of the input to the sampling means corresponds to said threshold.
In this case, the filter means allows the transmitted pulses received directly from other systems to reach the processing means, but ensures that those signals are inhibited so as to cause less serious interference.
The level of the threshold is ideally such that all transmitted pulses received directly from other systems, but as few as possible of the genuine reflected pulses exceed the threshold.
In either case the filtering means preferably further comprises threshold setting means for adjusting the threshold applied by the threshold detector to ensure that the majority of genuine reflected pulses lie below the threshold.
Preferably, the threshold setting means is operable to analyse the statistical distribution of the amplitudes of the noise received by the receiving means at a plurality of sample times, and to set the threshold at a level which is dependent upon the spread of the distribution.
To that end, the threshold setting means is preferably connected to the output of the sampling means.
In that connection, the threshold setting means may be so arranged as to set a threshold which is approximately three standard deviations above the mean of said distribution.
The system may be so arranged that if the statistical behaviour of the noise exhibits a given characteristic a warning signal is generated.
Thus, the threshold setting means can take account of variations in the background noise levels which might otherwise result in the application of a threshold which is too high or too low.
The system may be adapted for installation on a motor road vehicle, and may be operable to detect the approach of objects or other vehicles to said vehicle, the system further including alarm means connected to the signal processing means for warning the driver of any risk of collision with said obstacles or other vehicles.
According to a second aspect of the invention, there is provided an electromagnetic sensor system comprising transmitting means for transmitting a train of radio frequency pulses, receiving means for receiving reflections of said pulses from remote objects, sampling means for sampling the output of the receiving means during each of a succession of sampling periods, processing means for analysing the output from the sampling means to provide an indication of at least the presence of any such object and its distance from the receiving means, the system also including filtering means for detecting intermittent continuous wave radio frequency signals of frequencies which would cause interference in the sampler output and, in response to any such detection, preventing or inhibiting the operation of the system so that said continuous wave signals do not give rise to spurious detections of objects or incorrect indications of distances by the processing means.
This aspect of the invention exploits the fact that many radio communications systems use non-continuous transmission. For example, a mobile phone operating in accordance with a Time Domain Multiple Access (TDMA) protocol operates with a 1-in-7 duty cycle at 270 Hz. Thus, if the mobile telephones"" allocated radio channel falls into the operational band of the impulse electromagnetic sensor system, the filter means will. in effect, blind the system for approximately 4 ms in every 30 ms. This corresponds to relatively small loss of information which can be readily accommodated by the processing means.
The filtering means may be arranged Lo inhibit the operation of the system whenever RF energy of a frequency lying in a predetermined band is detected. Alternatively, the filtering means may be arranged to determine the periodic energy pattern in such an RF signal, and to inhibit the system when interference is due to be present.
Preferably, the filtering means is operable to inhibit the operation of the system by interrupting the operation of the sampling means, or limiting the amplitude of the output sampling means.
Preferably, the filtering means is connected to the receiving means and is operable to detect said continuous wave RF signals by monitoring the output to the receiving means.
The system may include delay means connected in series between the sampling means and the receiving means, and downstream of the connection of the filtering means, in order to synchronise the operation of the filtering means with the supply of the signal to the sampling means.
However, the aforementioned feature can be dispensed with where the filtering means determines the frequency of occurrence of the continuous wave signal and in effect anticipate when the signal will be present.
Conveniently, the filtering means comprises integration means for determining the total energy of RF signals, within said band of frequencies, received by the receiving means over a given integration period, the filtering means being operable in response to an increase in said detected energy, said increase being indicative of the presence of a continuous wave interference signal.
According to a third aspect of the invention there is provided a method of operating an electromagnetic sensor system, the method comprising the steps of transmitting a train of radio frequency pulses at a given pulse repetition frequency, causing a sampler to sample the output of a receiver at each of a succession of sampling periods occurring at a sampling frequency which is a multiple of the pulse repetition frequency, forming from the output of the sampler a first channel in the spectrum of which any continuous wave interference radio signal of a frequency which is a multiple of the sampling frequency and/or the pulse repetition frequency is aliased to a band in the sampler output in which the reflected signals appear, forming a second channel in the spectrum of which said continuous wave interference signal is either aliased into a higher band or into the same band as in the first channel, depending on the frequency of the interference signal, and in the former case outputting a signal by filtering the second channel to remove any such interference signal and in the latter case outputting a signal by combining the first and second channels in such a way as to cancel out any such interference signal appearing in said band of the reflections.
In practice, the reflected signals would appear in a number of bands in the sampler output, including a band from DC to a relatively low frequency (for example 3.3 kHz). A continuous wave interference signal of a radio frequency which is an odd multiple of the sampling rate will be aliased to a much higher band of frequencies, and can therefore be removed by low pass filtering of the second channel. However, a continuous wave interference signal of a frequency which is an even multiple of the sampling rate will be aliased to the same low frequency band. However, such since a signal also appears in the first channel, it can be cancelled out by, for example, subtraction of one channel from the other (if the amplitude of the aliased signal is substantially the same in each channel).
Preferably, the step of forming the first channel comprises selecting samples only for periods in which any reflections of transmitted pulses in the range of the system cannot be received, the second channel being formed from all the samples produced by the sampler.
Where the channels are to be processed digitally, preferably the forming of the first channel further comprises a step of creating additional samples by a process of interpolation from the selected samples so that, the first and second channels have the same sampling rate.
Conveniently, the sampling rate is twice the pulse repetition frequency, the first channel being formed from samples obtained during alternate sampling periods.
Preferably, the method comprises the further steps of generating a third channel by subtracting the first channel from the second channel, comparing the amounts of energy in the second and third channels, and forming an output signal from which whichever of those channels has the lower energy.
According to this aspect of the invention, there is also provided an electromagnetic sensor system for performing the aforementioned method, the system comprising transmitter means for generating a series of radio frequency pulses at a given pulse repetition frequency, receiving means for receiving reflections of said pulses from objects in the remainder of the system, sampling means for sampling the output of the receiving means at a rate which is a multiple of the pulse repetition frequency so that some of the samples occur periodically when reflections of the transmitted pulses in the range of the system are not received by the receiving means, signal processing means for forming a first channel from said some samples, a second channel from all the samples in the output of the sampler, and a third channel formed by subtracting the first channel from the second channel so as to remove any continuous wave interference signal which has been aliased in the output of the sampling means to the band in which any reflections are present, the apparatus further comprising comparator means for determining which of the second and third channel contains the least energy and forming an output from that channel.
According to a fourth aspect of the invention, there is provided an electromagnetic sensor system comprising transmission means for transmitting a train of radio pulses, receiving means for receiving reflections of said pulses, sampling means for sampling the output of the receiving means over a succession of sampling periods, each corresponding to a respective pulse, detection means for detecting the presence of any continuous wave radio interference signal and control means for so controlling the pulse repetition frequency and sampling rates at any such interference signal is aliased, in the output of the sampling means, to a band of frequencies outside the band in which said reflections appear.
Preferably, the control means is operable to control the sampling means and the transmission means in such a way that the sampling rate is the same as the pulse repetition frequency, the control means also being operable to select said frequency so that the interference signal is aliased to a higher band of frequencies than that in which the reflections appear.
In this case, the apparatus preferably includes a low pass filter for removing said interference signal from the sampler output.
According to this aspect of the invention, there is also provided a method of operating an electromagnetic sensor system, the method comprising the steps of transmitting succession of radio pulses, sampling the output of a receiver, arranged to receive reflections of said pulses, over a succession of sampling periods at a given sampling rate, determining whether the receiver has received any continuous wave interference signal which would be aliased into the band of the sampler output in which said reflections appear and, if such a signal has been detected, altering the pulse repetition frequency and sampling rate so that the interference signal is aliased to a band outside that in which the reflected signals appear.
Preferably, the pulse repetition frequency and sampling rate are chosen so that any such interference signal is aliased substantially to the Nyquist frequency in sampler output.
Preferably, however, the interferer is aliased to a frequency just below tile Nyquist frequency so that at least a first harmonic of the interference signal (and preferably also the second and third harmonics) also lie outside the band in which the reflected signals appear.
The pulse repetition frequency and sampling rate can be controlled to any value in a continuum. Alternatively, the pulse repetition frequency and sampling rate can be a selected one of a number of predetermined frequencies for any given band of interference signal.
According to a fifth aspect of the invention, there is provided an electromagnetic sensor system comprising transmitting means for transmitting a train of radio pulses. receiving means for receiving reflections of those pulses, sampling means for sampling the output of the receiving means over a succession of sampling periods, and processing means for processing the output of the sampling means to determine information on the presence or range of objects from which the pulses are reflected, wherein the sampling means comprises a plurality of samplers and filtering means for band-pass filtering the signal received by the receiving means so that each sampler is supplied with an RF signal lying in a respective one of a number of frequency bands, the sampler being operable to select for analysis the sampler output which has the lowest energy.
The invention also lies in a vehicle fitted with a system in accordance with any of the preceding aspects of the invention.